Improved Search for a Light Sterile Neutrino with the Full Configuration of the Daya Bay Experiment

This Letter reports an improved search for light sterile neutrino mixing in the electron antineutrino disappearance channel with the full configuration of the Daya Bay Reactor Neutrino Experiment. With an additional 404 days of data collected in eight antineutrino detectors, this search benefits from 3.6 times the statistics available to the previous publication, as well as from improvements in energy calibration and background reduction. A relative comparison of the rate and energy spectrum of reactor antineutrinos in the three experimental halls yields no evidence of sterile neutrino mixing in the $2\times {10}^{-4}\lesssim |\mathrm{\Delta }{m}_{41}^2|\lesssim 0.3{\mathrm{eV}}^2$ mass range. The resulting limits on ${\sin }^{2}2{\theta }_{14}$ are improved by approx imately a factor of 2 over previous results and constitute the most stringent constraints to date in the $|\mathrm{\Delta }{m}_{41}^2|\lesssim 0.2{\mathrm{eV}}^2$ region.

Figure 1

Layout of the Daya Bay Reactor Neutrino Experiment. The dots represent reactor cores, labeled as D1, D2, L1, L2, L3, and L4. The Daya Bay experiment started data taking with six antineutrino detectors (AD1–AD6) installed in three experimental halls (EH1–EH3). From August to October 2012, two additional detectors (AD7 and AD8) were installed in EH3 and EH2, respectively.

Figure 2

Prompt energy spectra observed at EH2 (top) and EH3 (bottom), divided by the prediction from EH1 with the three-neutrino best fit oscillation parameters from the most recent Daya Bay analysis [31]. The gray band represents the one-standard-deviation uncertainty of the three-neutrino oscillation prediction, which includes the statistical uncertainty of the EH1 data, as well as all the systematic uncertainties. Predictions with ${\sin }^{2}2{\theta }_{14}=0.05$ and two representative $\mathrm{\Delta }{m}_{41}^2$ values are also shown as the dotted and dashed curves.

Figure 3

Exclusion contours in the (${\sin }^{2}2{\theta }_{14}$, $|\mathrm{\Delta }{m}_{41}^2|$) plane, under the assumption of $\mathrm{\Delta }{m}_{32}^2>0$ and $\mathrm{\Delta }{m}_{41}^2>0$. The red long-dashed curve represents the 95% CL exclusion contour with the Feldman-Cousins method [40] from method A. The black solid curve represents the 95% ${\mathrm{CL}}_s$ exclusion contour [41] from method B. The expected 95% CL $1\sigma$ band in yellow is centered around the sensitivity curve, shown as a thin blue line. The region of parameter space to the right side of the contours is excluded. For comparison, Bugey’s [43] 90% CL limit on ${\overline{\nu }}_e$ disappearance is also shown as the green dashed curve.